[callmejoe]

an avalanche starts with a snowball . . .
the water . . .the water . . .the water . . .

http://news.bbc.co.uk/1/hi/sci/tech/956280.stm

http://edition.cnn.com/2001/TECH/science/08/29/tidal.wave/

http://www.bbc.co.uk/science/horizon/2000/mega_tsunami.shtml

[Godzilla]

an avalanche starts with a snowball . . . the water . . .the water . . .the water . . .

Key point that I was making - Something far more substantial than even the biggest nuke is necessary. Earthquake/volcanic is also a source, but was trying to keep the example simple.